EXTRUSION: Hydraulically Powered Backflush Screenchanger

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An automated, hydraulically powered backflush system enables the V-Type screen changer from Nordson Kreyenborg (U.S. office Roswell, Ga.) to purge even heavy contaminant from the melt filter element while maintaining extrusion throughput, minimizing operator intervention, and avoiding downtime in constant-pressure processes like film and fiber extrusion or strand pelletizing

In this screen changer, melt flow from the extruder splits into four streams for filtration in two pairs of screen cavities, then the streams rejoin with no significant change in melt flow. Each pair of cavities is mounted in a piston which positions both cavities so that they can filter their respective melt streams, or removes one of them from the process to remove contaminant buildup by means of backflushing. In normal operation, polymer is flowing through all four cavities. While one of the cavities is changed, the other three remain in the process.

In addition to the two screen-bearing pistons, there are four hydraulically actuated displacement pistons that operate during backflushing. When the differential pressure across the screen changer increases to a pre-set level because of contaminant buildup, the backflush sequence for all cavities will be started automatically. For each cavity, the corresponding displacement piston on the outlet side of the screen retracts, creating a reservoir of filtered molten polymer. This material is hydraulically compressed and discharged in reverse direction, back through the screen, carrying away contaminant for removal from the system. The sequence is performed for each cavity one after the other.

This “power backflush” technology efficiently purges filter media even in extrusion lines running highly viscous polymers; minimizes the amount of material consumed in backflushing; and lengthens the working life of filter media, the company says. Rheologically optimized flow channels avoid excessive residence time and stagnant zones in the system.

As in the case of backflushing, the process for changing screens is initiated when a pre-defined number of backflushes is reached. This triggers the outward movement of the screen-bearing piston so that the screen pack can be removed and a new filter element is put in its place. Also as in backflushing, three of the cavities remain in the process while a new screen pack is installed in the fourth.